1. Field of the Invention
This invention relates to an LaGaO3-based sintered body (hereinafter LaGaO3 sintered body). More particularly, it relates to an LaGaO3 sintered body which has high mechanical strength and excellent electrical conductivity and is therefore applicable to practical big-volume products. The LaGaO3 sintered body of the invention is useful as an oxygen-permeable membrane, a reactor, a sensor device, etc.
This invention also relates to an LaGaO3-based sintered body which is suited for use in, for example a sensor device and a process for producing the same.
This invention further relates to a sensor device using an oxide ion-conducting solid electrolyte. More particularly it relates to a critical current sensor device which has improved adhesibility between solid electrolyte and electrode, which has reduced solid electrolyte/electrode interfacial resistance by increasing three-phase interface comprising gas phase, electrode and solid electrolyte between solid electrolyte and electrode, and therefore exhibits improved oxygen pumping ability.
2. Background Art
It is known that an LaGaO3 sintered body is far more electrically conductive than stabilized zirconia and can be used as an electrolyte of a solid electrolyte fuel cell that exhibits excellent power generation properties in a low-temperature range as disclosed in JP-A-9-161824. However, the problem of the LaGaO3 sintered body of the background art is its low mechanical strength. For example, the bending strength of yttrium-stabilized zirconia (hereinafter referred to as YSZ) as obtained by sintering at 1500xc2x0 C. is 500 MPa, while that of LaGaO3 sintered body is as low as 200 MPa or even lower. Therefore, in order to apply LaGaO3 sintered body to practical products, particularly big-volume products, it has been necessary to add extensive improvements in mechanical strength.
An oxygen sensor device having an oxide ion conductor typically uses stabilized zirconia solid electrolyte as an oxide ion conductor. Such an oxygen sensor device has been used in practice as, for example, an oxygen sensor for automotive engines. Zirconia oxides have also been used in broad fields as fuel cells, reactors, etc. because of their chemical stability and high oxygen conductivity.
The exhaust gas sensor disclosed in JP-A-9-311120 can be mentioned as an example of sensor devices using a zirconia oxide. According to the disclosure, an oxygen pump cell comprising an oxide ion-conducting solid electrolyte is operated in such a manner that an oxygen sensor cell comprising an oxide ion-conducting solid electrolyte which is placed in a detection chamber may give constant signals (the electromotive force of an oxygen concentration cell) and a component of an exhaust gas is detected from a resistivity change of a semiconductor detector placed in the detection chamber.
However, in detecting hydrocarbons (HC) with the above-described sensor device, hydrocarbons tend to react with oxygen and decompose (i.e., the concentration of the component to be detected decreases) by the catalytic action of the noble metal electrodes used in the oxygen pump cell and the oxygen sensor cell, resulting in reduction of detection accuracy. If at least the electrodes of the oxygen pump cell and the oxygen sensor cell that face the detection chamber are made of a material which is catalytically inert to hydrocarbons, hydrocarbons will hardly react and decompose in the detection chamber, whereby accurate detection of hydrocarbons could be achieved.
In order to achieve high oxide ion conduction by use of a zirconia oxide, the working temperature must be as high as 700xc2x0 C. or even higher because the zirconia oxide itself does not exhibit high oxide ion conductivity at low temperature. Further, the interfacial resistance between the electrodes (e.g., Pt electrodes or Au electrodes) and a zirconia oxide is high due to poor adhesion to each other.
Most of hydrocarbons will be burnt at such a high working temperature as 700xc2x0 C. or higher and are no more measurable whether or not the electrodes are made of a material catalytically inert to the hydrocarbons. Additionally, considering the sensing system as a whole, high power consumption arising from the high working temperature is problematical.
An LaGaO3 oxide is known as an oxide ion-conducting solid electrolyte that works at lower temperatures than a zirconia oxide. However, it has been pointed out that an LaGaO3 oxide is reactive with a noble metal, particularly Pt, so that a sensor device comprising a Pt electrode in combination with an LaGaO3 oxide has an increased interfacial resistance, failing to perform a high oxygen pumping function. It is necessary to use a noble metal, which is hardly oxidized even in high temperature, as an electrode material in sensor devices used at 500xc2x0 C. or higher. Hence, the reactivity of an LaGaO3 oxide with a noble metal has been a hindrance to application of this oxide to a sensor device.
Yttrium-stabilized zirconia is known as a solid electrolyte useful in a sensor, etc. and has been used widely. The problem of YSZ used in a sensor is that its oxide ion conductivity drastically reduces in low temperature so that the working temperature of the sensor should be high enough in order to obtain high ion conductivity.
A sintered body of lanthanum-gallium mixed oxide having a perovskite structure, i.e., lanthanum gallate-based sintered body (LaGaO3 sintered body) has recently been attracting attention as a substance showing higher oxide ion conductivity than YSZ and been given much study.
The LaGaO3 sintered body comprises LaGaO3 with part of La or Ga displaced with a less valent atom, such as Sr or Mg, by substitutional solid dissolution to have increased oxide ion conductivity.
However, it is difficult to obtain a dense LaGaO3 sintered body. Even through firing is performed at 1500xc2x0 C. or even higher temperatures, the resulting sintered body tends to have gathered pores.
An object of the present invention is to provide an LaGaO3 sintered body having markedly improved mechanical strength without greatly impairing the high electrical conductivity inherent thereto.
An object of the invention is to provide a sensor device which has a markedly reduced interfacial resistance between an oxide ion-conducting solid electrode and a noble metal electrode and therefore exhibits greatly improved oxygen pumping ability and works satisfactorily even in temperatures of 700xc2x0 C. or lower.
An object of the present invention is to provide a dense LaGaO3 sintered body having a high density and a process for producing the same.